The present invention relates generally to processing of wastes by filtration. In particular, the present invention relates to the processing of radioactive waste streams at nuclear facilities such as nuclear power plants to separate the contaminants from an aqueous effluent.
In a nuclear power plant and at other manufacturing and processing facilities where nuclear materials are handled, the waste streams from the facility operations must be processed prior to discharge or reuse in order to remove radioactive contaminants for disposal. These waste streams come from a variety of sources, such as a spent fuel pool, floor drains, and resin tank drains. Because of increasing disposal costs, the separation of the contaminants from the water that carries them has become more and more important. The goal of this separation is to (1) remove sufficient contamination from the aqueous waste stream so that the resulting effluent can be reused or released to the environment and (2) reduce the volume of the waste that must be disposed of.
A number of techniques are used to separate wastes from waste streams, including filtration and ion exchange. Because the wastes may be in the form of particulate of varying sizes and in the form of dissolved ions, these two techniques are commonly used in a particular sequence. Filtration is essentially the removal of particulate from water by passing the water through a porous structure leaving the particles, whose passage is blocked by the structure, behind. Sometimes more than one type of filter is used. By using ever finer filters, including mechanical filters, microfilters, ultra filters, and nano-filters, a very high percentage of particulate can be removed. These various filtration devices are used in a specific sequence; the coarser filters being used first to remove the larger particulate. Then finer filters are used to remove the smaller particulate.
This approach makes good sense. If a fine filter is used first, the amount of particulate it would remove would be so great that the filter, blinded with particulate both fine and coarse, would stop the flow altogether soon after being placed in service. Using filters in sequence from coarse to fine assures that the throughput of each filter is as high as possible. Furthermore, because filters with smaller pore size are generally more expensive, it makes economic sense to use fine filters only for filtering the smallest particles and not also particles that could be filtered with less expensive filters.
From the finest filters used, the waste stream is routed to an ion exchange bed and/or reverse osmosis membranes where dissolved contaminants can be removed. The resulting effluent is nearly free of both particulate and dissolved solids.
Referring to FIG. 1, which illustrates the current practice in a nuclear power plant for handling aqueous waste streams, the aqueous wastes are collected in a waste holdup tank 10. Heavier particulate will settle in the tank, the waste containing suspended particulate is directed through an ultrafiltration membrane 20. Ultrafiltration membrane 20 generally has the capability of ejecting particles of about 0.005 microns or larger at an operating pressure of 3-10 bar. Low molecular weight substances such as sugars and salts pass through it. The effluent from ultrafiltration membrane 20 is next directed through a reverse osmosis membrane 30 or an ion exchange bed 40. Essentially, only water passes through reverse osmosis membrane 30 at its normal operating pressure range of 20-60 bar. The effluent from reverse osmosis membrane 30 or ion exchange bed 40 will be highly purified.
The wastes from each of these steps: the settled particulate from waste tank 10, the rejects from ultrafiltration membrane 20 and reverse osmosis membrane 30 and, eventually, the resins from ion exchange bed 40 will be subjected to further processing 60 to stabilize them for disposal in various ways, including drying and/or solidifying them in a cementitious medium.
The process just described works well. It produces a very clean effluent and the waste itself can be disposed of safely. However, it focuses solely on obtaining a clean effluent. As disposal prices have continued to climb, there has been a growing need to reduce the volume of wastes being disposed of. Thus, there is a need for a way to process wastes that results in less volume and easier handling but does not compromise the quality of the effluent.
According to its major aspects and briefly recited, the present invention is a method and apparatus for processing aqueous waste streams, especially waste streams from nuclear power plants. The present process reduces the volume of waste to be disposed of compared to the prior art process without affecting the purity of the effluent. It also simplifies handling of the wastes.
In order to achieve this reduction in volume of waste, a portion of the reject from the ultrafiltration step described above is processed in an additional filtration step. The effluent from this additional step is returned upstream of the ultrafiltration membrane and xe2x80x9crecycledxe2x80x9d; that is, it again repeatedly directed to the ultrafiltration membrane and then through the additional filtration step. The concentrate from this additional step may then be disposed of directly after de-watering.
The additional step includes passing a stream of concentrate collected by the ultrafiltration membrane through a microfilter at a low flux. The velocity through the filter is maintained very low to maximize the solids loading of the microfilter. As solids are collected on the microfilter, the pressure drop will eventually increase. The filter will be replaced when either the maximum allowable pressure drop across it, or, in the case of filtration of radioactive contaminants, the maximum allowable radiation dose is reached.
The microfilter removes substantially all the particulate, including particles smaller than its pore size, because, for all practical purposes, it has a nonzero particle removal efficiency over the range of particle sizes in the waste stream. As long as the particle removal efficiency is greater than zero, the filter will eventually, after repeated passes, remove substantially all particles. By recycling the microfilter effluent to the ultrafiltration membrane, the concentration of the particles in the recycle loop will build up to a level where the rate of particle removal in one pass through the microfilter is equal to the rate of particles being introduced with the feed into the ultrafiltration membrane, establishing an equilibrium.
One contributing factor as to why the microfilter will remove particles smaller than its pore size is due to particle agglomeration; however, agglomeration is just one of the reasons that particles are removed by the microfilter (in fact, agglomeration is not necessary for the microfilter to work as long as other particle removal mechanisms produce a particle removal efficiency that is greater than zero). Agglomeration is a tendency of the particles to form clusters that interlock. The interlocking clusters define narrow, twisting passages through a cake-like matrix that allow water to flow. These passages are irregular, that is, they change direction and cross section, resulting in a filtering action that will trap particulate including particulate that would otherwise pass through the microfilter.
Other forces, such as adsorption, contribute to the removal of small particles. The formation of the cake of particulate against the upstream side of the filtration medium also contributes to the removal of small particles. The cake in effect becomes part of the filter. Cake formation reduces filter pore size and increases the depth of the filter, increasing the likelihood that a small particle will become trapped or adsorbed by the filter. The low flux helps to are that the forces acting on particles from fluid flow do not disrupt the adsorption forces or break up particulate aggregates.
Additionally, the removal efficiency of the Ultrafiltration (xe2x80x9cUFxe2x80x9d) system for radioactive contaminants can be improved by precipitation of radioactive metals dissolved in the waste water. By the addition of sulfides or other chemicals (e.g., other precipitating agents or pH adjusting chemicals) upstream of the ultrafiltration membrane, the metals will be precipitated before they would otherwise pass through this membrane. They, along with other particulate, can be accumulated and concentrated on the microfilter. This precipitation of metals can be so effective that further treatment downstream of the ultrafiltration membrane, as described above, can be eliminated, thus saving all costs associated with that equipment.
The present invention has a number of advantages over other systems. In particular, it is a filtration system for total suspended solids removal that processes a waste stream to produce a effluent (filtrate) stream sufficiently clean for release or re-use and segregating and accumulating the solids into a device, namely, a microfilter, suitable for disposal.
The present system avoids the generation of a reject stream that, in the prior art, requires further processing, such as by drying, solidifying, settling, and centrifuging. Avoiding these processes avoids the corresponding effort, expense, and exposure attendant to them.
The present system allows the effective separation of the collection of wastes from the two primary reject streams, one that primarily contains suspended solids and the other that primarily contains dissolved solids. This separation opens the door for independent and specific treatment of each reject stream. In fact, the treatment that is applied in the present invention results in the elimination of the reject stream that contains suspended solids from an ultrafiltration membrane that in prior art require further processing.
The present system uses the delivery of a portion of the reject from an ultrafiltration membrane by a slip stream to a microfilter, as an integral part of the processing system, notwithstanding the fact that the microfilter has a micron rating larger than the pore size of the ultrafiltration membrane.
The present system, which relies heavily on mechanical filtration through a microfilter, successfully removes substantially all particulate, notwithstanding the fact that the microfilter has a removal efficiency of less than 100% per pass. This success is achieved by repeated recycling of the clarified effluent from the microfilter to the reject side of the ultrafiltration membrane in combination with the low flux across the microfilter.
In the present invention, the use of a low flux slip stream from the reject side of the ultrafiltration membrane maximizes the loading of solids in the microfilter and, consequently, minimizes the frequency of filter change-out and provides numerous other benefits, such as lower dose to personnel and lower material costs.
The present invention also takes advantage of chemical treatment, e.g., pH adjustment or addition of precipitating agents such as sulfur-containing compounds, to the feed to the ultrafiltration membrane in order to precipitate dissolved contaminants and, consequently, to maximize removal of contaminants in feed and make it possible to discharge or reuse the ultrafiltration membrane effluent without the need for further processing steps, such as reverse osmosis or ion exchange.
Because the capacity of the present system can easily be expanded, it can be configured to meet individual plant""s wastewater characteristics and existing systems. Furthermore, it can be installed easily in current plants"" wastewater treatment systems.
The ultrafiltration membrane can be back flushed to minimize chemical cleaning requirements. Back flushing is performed to remove deposits from the membrane surface in order to recover membrane flux. Back flushing efficiency can be improved by using the reject recirculation pump to produce a high axial velocity through the membrane during back flushing. A gas, such as air or ozone, could be injected either into the backflush liquid or into the recirculating reject stream during membrane back flushing in order to improve back flushing effectiveness by increasing turbulence or by chemically reacting with foulants.
The present invention is being described with frequent reference to radioactive contaminants generated at nuclear power plants. However, it is applicable to all waste water systems where particulate and dissolved materials that can be precipitated must be removed from an aqueous waste stream.
Another important feature of the present invention is the use of a microfilter to receive reject from an ultrafiltration membrane. This feature is counter-intuitive because, in the prior art, waste streams are typically passed through a series of filters beginning with coarser filters and proceeding through finer filters.
A feature of the present invention is the use of a low flux through a microfilter to trap particles, including those smaller in size than the micron rating of the filter. This combination allows greater levels of filtration at cost effective rates than would otherwise be achievable only by using more expensive filters.
Still another feature of the present invention is the use of sulfides or other chemicals to precipitate dissolved metal ions prior to the metal ions reaching the ultrafiltration membrane so that the metal ions can be removed along with particulate. The resulting water passing through the ultrafiltration membrane may well be pure enough to meet release criteria.
Other features and their advantages will be apparent to those skilled in the art of filtration from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following Drawings.